A conventional load sensor disclosed in Japanese Patent Laid-Open Publication No. 09-254778 will be described below.
FIG. 8 is a sectional view of the conventional load sensor. The conventional load sensor mounted in a car body 23 includes a first spring seat 18, a second spring seat 19, a conical spring 15, a cylindrical spring 16, a piezoelectric element 17, a pedal 21, and a brake arm 22. The conical spring 15 is located between the second spring seat 19 and the first spring seat 18 attached to the brake arm 22. When a driver starts to step on the pedal 21, a load is applied to the piezoelectric element 17 via the conical spring 15, and the piezoelectric element 17 converts the load to an electric signal corresponding to the load by its piezoelectric effect and detects the load. FIG. 9 shows a load characteristic of the sensor. In FIG. 9, the horizontal axis represents a stroke of the pedal, and the vertical axis represents the load. When the stroke is in a range L, the load and a reaction by the conical spring 15 are balanced. When the stroke exceeds the range L, the first spring seat 18 contacts the cylindrical spring 16, and then, the load and the sum of respective reactions by the conical spring 15 and the cylindrical spring 16 are balanced. The load characteristic of the load sensor has a fold point P, as shown in FIG. 9.
FIG. 7 is a load characteristic diagram of a hydraulic brake. The horizontal axis represents a stroke of a pedal, and the vertical axis represents a load. In the hydraulic brake, as shown in FIG. 7, the load rapidly increases when the stroke of the pedal exceeds a predetermined value.
The fold point P is not found in the load characteristic diagram of the hydraulic brake in FIG. 7. A rapid change of the load at the fold point P of the conventional load sensor provides an uncomfortable pedal feeling, while the hydraulic brake does not provide an uncomfortable pedal feeling.